A proximity switch that exchanges information for identification by carrying out a non-contact communication between the sensor unit and the actuator unit according to the principle of electromagnetic induction is being developed as a proximity switch used for safety management purposes such as detecting the opening and closing of doors to dangerous regions (see patent documents 1, 2).
Specifically, patent document 1 describes arranging a memory for storing identification code in the actuator unit (operation element) and the sensor unit (reading head), so that the sensor unit communicates with the actuator unit to read out the identification code on the actuator unit side and checks such identification code with the identification code in the memory of its own circuit to determine the presence or absence of the actuator unit (see pages 7 to 9, FIG. 2 of patent document 1).
Patent document 2 discloses a proximity switch having a configuration in which a transmitting antenna coil and a receiving antenna coil are arranged in both the sensor unit and the actuator unit, so that signals of different frequencies are transmitted and received between the antenna coils. In such proximity switch, an externally input digital code signal is modulated by a predetermined frequency to be transmitted to the actuator unit in the sensor unit, and the signal received by the actuator unit is decoded to a digital code signal and then such digital code signal is modulated by a frequency different from that of the signal from the sensor unit to be transmitted to the sensor unit. In the sensor unit, the signal transmitted from the actuator unit is decoded to the digital code signal, and the presence or absence of the actuator unit is determined by whether or not such signal matches the original digital code signal. The frequencies of the signal transmitted from the sensor unit and the signal transmitted from the actuator unit are set to be different so that each signal can be transmitted without interfering with the other (see paragraphs 0023 to 0036, FIGS. 1 to 3 of patent document 2).    Patent Document 1: Japanese Patent Publication No. 2002-501698    Patent Document 2: European Patent Publication EP1181776 B1